1. Field of the Invention
This invention relates to a radar processing system and particularly to a system for automatically focusing a synthetic array through derivation of focus error signals representative of required parameter changes in the processing of synthetic array image data.
2. Description of the Prior Art
Synthetic array processing requires that a quadratically varying phase correction i.e., an azimuth focus reference function be applied to the pre-summed sequentially derived signal vectors before integrating them, with (or without) amplitude weighting, to obtain each resolved element's amplitude. This quadratic azimuth focus reference function is calculated as a function of range, velocity, look angle and wavelength. Multiplication of successively derived signal vector returns from a particular ground scatterer by the quadratic focus reference function, and integration of the resulting products is in essence the correlation process. Many parameters may affect the accuracy of this quadratic phase correction such as a change in direction of the aircraft deriving the data, variations in the height of ground features, and errors in the velocity data which, even when derived from inertial platforms, may be only marginally accurate to achieve optimal focus. There are several known schemes for detecting cross line of sight velocity error or defocusing error, none of which have been found to be substantially satisfactory. One conventional method of correcting velocity data measures the target drift in successive tracking telescope synthetic array images; another scheme utilizes additional correlation filters i.e., outrigger correlation channels whose relative output amplitude is compared to derive a measure of defocus. In the latter scheme an overfocus correction is applied in one outrigger channel and an underfocus term is applied in the other. Magnitude detection is performed in each outrigger correction channel. The resultant signal vector magnitudes are then differenced to extract the magnitude and sign of the focus error. The former scheme is only suitable for a tracking mode of operation, both schemes have been found to be complex to implement digitally, and the latter has been found to suffer from focus error data contamination by off-axis targets i.e., ground returns not at the correlation peak.